Imagery collected using visual sensors, such as charge-coupled devices (CCDs), are prone to non-uniformities and noise resulting from changes in the CCD response due to non-standard operating conditions. One type of non-uniformity may include incorrect sensing of object irradiance, for example. This incorrect sensing may be a caused by non-standard operating temperatures at the focal plane of the CCD and/or an analog processing unit of the imaging sensor. There may also be a thermally induced leakage current on the CCD as a result of temperature variations from a non-standard condition.
Accordingly, changes in temperature may cause undesirable changes in operation of the CCD and the analog processing unit of the imaging sensor. In certain applications, such as multi-spectral imaging, there may be a need to perform computer analysis on image data in addition to forming pictures. In order for the analysis to proceed correctly, a faithful reproduction of the real image needs to be provided. Any changes observed in the image should reflect changes that occurred at the target site of the imaging sensor and not in the image processing chain (e.g., CCD focal plane, analog processing unit, etc.).
Generally, remote sensing systems are calibrated to obtain the dark response and radiometric response of the sensor close to the time of image acquisition. However, variations in temperature and radiation levels cause the sensor response to change. Accordingly, the sensor response must be monitored frequently and monitored at a time close to image acquisition.
Furthermore, calibration of the sensor is performed by changing one or more operational characteristics of the sensor, such as resistance of the sensor, DC bias current applied to the sensor, and AC bias waveform applied to the sensor, for example (see US Patent Pub. No. 2002/0074491 A1 to Butler). This presents undesirable complexities in sensor manufacture and operation, as the added control components may fail during operation. Additionally, typical imaging systems close their shutters in order to obtain the dark response for each calibration cycle, thereby undesirably extending the time period between calibration and image acquisition. This results in systems with fast shutter speeds that are expensive, and inexpensive systems that have undesirably slow shutter speeds.
In some remote sensing systems, the temperature of the focal plane and electronics is controlled, for example, by a thermoelectric cooler to stabilize the sensor response and minimize changes due to system temperature variations. However, in order to achieve an accurate and stable sensor response, the thermal environment needs to be controlled within a small temperature range. Precise environmental temperature control requires additional power (e.g., for heating and cooling), thereby adding undesirable weight to the imaging sensor package. Furthermore, it is impractical to obtain precise temperature control over imaging sensors that are situated in outer space.